In many cases, it is desirable to identify what chemical species are in a gas mixture. One way to achieve this is through spectroscopy. In spectroscopy, each chemical species can interact with incident radiation in both linear and nonlinear ways to produce a spectrum that depends on the structure of that species. The spectrum can be an absorption spectrum or an emission spectrum. In either case, it often includes narrow features, namely absorption or emission spectral lines at particular well-defined frequencies. In a mixture including two or more non-interacting species, the spectrum of the mixture is a superposition of the spectra of the individual species. Thus, by inspecting the spectral lines of a mixture, one can in principle identify its constituent species.
A difficulty that arises in practical systems is that spectral lines can have finite width. This can result from limits in instrument resolution or, in some cases, from fundamental properties associated with chemical species. Thus, in a mixture with many constituents, the separation between spectral lines of different species or of different states or conformers of the same species can be less than the line width. As a result, individual spectral lines may not be resolvable. This undermines the ability to identify the individual species within the mixture. In practice, this difficulty in resolving spectral lines impedes room temperature spectroscopy from being an effective tool for chemical analysis of complex mixtures of molecules including more than about eight atoms.
Cooling a molecular gas can greatly reduce the number of states a molecule occupies, which in turn simplifies its spectrum. More specifically, cold molecules can occupy far fewer rotational and vibrational states than warm molecules. Cold molecular gases therefore can have much simpler spectra including far fewer and, in some cases, narrower lines than corresponding spectra for warm molecular gases. As a result, spectroscopy can be used to identify larger molecules when a mixture of the molecules is cold rather than when the mixture is warm. For the foregoing reasons, it is desirable to deliver cold gas phase molecules to a spectrometer.
It is against this background that a need arose to develop the embodiments described herein.